Project Summary With 382 billion data points, NHGIS is the largest publicly accessible population database in the world and is an essential component of the shared data infrastructure for population and health research. Over the past 16 years, NHGIS has collected aggregate U.S. census data from diverse sources, formatted them consistently, developed machine-readable documentation, and created high-precision boundary files describing census geographic units over time. To eliminate major obstacles for studies of small-area population change, NHGIS has developed geographic time series that link comparable data across multiple census years. The data are broadly accessible to health researchers through powerful dissemination tools that make it easy to navigate the intricacies of the U.S. statistical system. The availability of large-scale integrated census data has opened exciting new opportunities for fine-grained contextual analyses of population dynamics and health. These data are essential for spatial analysis of mortality and disease incidence and for assessing the impact of neighborhood characteristics on health and well-being. NHGIS reduces costs for population and health researchers by minimizing redundant effort, simplifying data access, and improving data reliability. Over the past five years, the number of NHGIS users has increased 250%, and the project now disseminates seven terabytes of data per year. To meet the demands of a rapidly increasing user base and ever-expanding requests for more and improved data, this project has five major goals: 1. Ingest of new census data. The quantity of data in NHGIS will expand dramatically over the coming five years to include tabular and spatial data from the 2020 decennial census and 2017-2021 ACS. 2. GIS boundary files for 1970 and 1980 census blocks. The project will create and disseminate boundary files delineating 1970 and 1980 census blocks, the finest-resolution units identified in census summary data. 3. New and extended spatiotemporal data tables. The project will (1) extend time series to 2020; (2) extend geographically standardized times series back to 1970 and 1980; (3) create spatial relationship files for standard census units over time; (4) create annual tract population estimates by sex and age, 1990-2020. 4. Data access tools. The project will (1) create and publish software to generate spatially consistent zones from any set of mismatched zones; (2) provide users with greater flexibility to select and obtain data for specific geographic extents; (3) design and implement continued interface and system refinements. 5. Dissemination, training and outreach. We will provide user support, webinars and outreach to users.